Perpendicular flap: Document a stronger coupling condition

[Fujikawas]

With this change the coupling becomes stronger for the perpendicular flap case. It will require implicit coupling AND acceleration methods to converge. With $\rho=42$, it requires only implicit coupling, no acceleration is necessary. With current $\rho=3000$, both explicit and implicit coupling could lead to convergence.
With $\rho=1$, the result from fluid-openfoam-solid-dealii looks like:

With $\rho=42$, the result from fluid-openfoam-solid-dealii looks like:

Checklist:

• I added a summary of any user-facing changes (compared to the last release) in the changelog-entries/<PRnumber>.md.
• I will remember to squash-and-merge, providing a useful summary of the changes of this PR.

[uekerman]

Worth considering on top: We could make the fluid inflow time-dependent. Then, also the $\rho=3000$ scenario could look interesting again.

[MakisH]

@Fujikawas please add in a comment the new figures you got in the context of https://github.com/MakisH/ofw19-training

[MakisH]

This has high educational value, as it demonstrates a case in which implicit coupling is important.
To not trigger a cascade of side changes that require a lot of effort, let's convert this to a documentation note, encouraging the user to try a different density and observe the effect. (@Fujikawas)

[MakisH]

I directly fixed a few issues and removed the intermediate step of ρ=42 and no acceleration, as that I think is more confusing than helps.

Current rendering: